Referring to FIG. 1, providing an example of a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow in the example shown into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 in the example shown before being exhausted through the nozzle 19 to provide additional propulsive thrust. It will be understood some engines do not have an intermediate compressor or intermediate turbine. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts.
Because the bypass ratio is high it will be appreciated that the total size of the fan blades utilised is high with respect to the compressor and turbine stages of a gas turbine engine. The number of blades will be in the order of twenty while there will be significantly more compressor or turbine blades. Traditionally, these blades were solid formed of a relatively lightweight metal for structural strength. More recently, hollow blades or blades filled with an appropriate viscous material have been specified and used. It will be appreciated that by providing a hollow or blade filled with a lighter weight material the ongoing objective of reducing engine weight is achieved. Nevertheless, such blades are susceptible to damage as a result of percussive impacts and operational ageing. There may also be problems with respect to localised plastic deformation of the blade which may cause flutter upon blade rotation. It will be appreciated that these problems persist whether the infill is a simple viscous material or a composite material. In view of the above ideally a blade should be lightweight for aircraft applications, whilst providing necessary structural strength and characteristics for blade operation and resistance to impact energies beyond acceptable levels.